Vinylcyclobutane-cyclohexene reaction in natural products

I was not aware of the really neat paper by Baran and Houk in which they justified a dicationic diradical mechanism for the rearrangement of skeptrin to ageliferin. This may be the final nail in the coffin of the earlier assumed 'hymenidin-ageliferin' rearrangment which was thought to be a DA reaction catalysed by a novel Diels-Alderase.



This was of course the sequel to Baran's remarkable 2004 paper in ACIE, in which he convincingly questioned the DA reaction and instead proposed a vinylcyclobutane-cyclohexene reaction. What I found neat was the simple reason why he questioned the hypothesis; when the natural products are isolated, ageliferin is always present in smaller amounts, whereas if both ageliferin and sceptrin were formed from hymenidin, then thermodynamically, ageliferin is more stable and hence it should be the major product. Ergo, the only way in which it can be the minor product is if it is derived from sceptrin through an energetically high VC-CH rearrangement.



Now, Houk's calculations seem to support the diradical mechanism. As a note, he finds UB3YLP more suitable for these calcs rather than the now popular mpW1 functional or the CASSCF methods.

Refs:
1. Baran, Houk, et. al. Angew. Chem. Int. Ed. Vol.43, 20 Pages: 2674-2677
2. Baran et. al. Angew. Chem. Int. Ed. Vol.45, 25 Pages: 4126-4130